1. Field of the Invention
The present invention generally relates to planing hulls for watercraft and, more particularly, to planing hulls for sailboards/windsurfers for improving the transition from displacement operation to planing operation and exhibiting increased speed over a wider range of wind speed.
2. Description of the Prior Art
Hulls of watercraft may be of either of two distinct types: a displacement hull which derives vertical lift from the weight of water displaced by the hull and a planing hull which derives vertical lift from thrusting water downwardly by the bottom surface of the hull when in motion. At rest or at low speed, planning hulls function in the same manner as displacement hulls. Displacement hulls are most efficient and derive greatest speed for a given amount of power if they are a long and narrow, streamlined shape. Planing hulls, on the other hand can be much more efficient than displacement hulls when planing and, since lift is derived from the angle of attack between the bottom surface of the hull and the water surface, are most efficient if wide and short; directly conflicting with the preferred shape for displacement hulls.
Therefore, in general, the more fully a hull is optimized for planing efficiency, the more power is required to reach planing speed. It follows that planing hulls must often represent a compromise between efficiency in the displacement and planing modes of operation, particularly where available motive power is limited such as when sails are employed. Conversely, wind/sail-powered watercraft such as sailboards generally operate well only within a narrow range of wind conditions.
More specifically, the area ratio or aspect ratio, AR, of a planing board relates the planing surface width to its length. The aspect ratio is given byAR=b2/area=b/c where “b” is the width of the planing surface, “area” is the planform area of the planing surface and “c” is the average length of the planing surface.
Narrow boards where the width of the planing part of the board is small, resulting in a small AR are generally thought to be faster than wider boards with a larger AR, possibly because they are more streamlined and easier to power at low speeds and achieve a more nearly optimum planing angle at high speeds even though the lift of the lower AR board may be only 60% to 70% of a board with twice the aspect ratio, AR, but the same area and planing angle. Once planing, however, a higher aspect ratio board can be faster either upwind or downwind.
For example, commercially available sailboards such as the Mistral Ultralight and the F2 race board are made for non-planing or marginal planing conditions and are long, narrow and streamlined but, as would be expected, do not plane well and are not as fast as planing “slalom” or short boards. For example, some boards like the commercially available Pro-Tech C. A. T. are wide and short and very fast when planing but comparatively slower at displacement operation speeds in light winds. Such short boards are also somewhat more difficult to control and “unfriendly” to inexperienced wind surfers. Other boards which are short and narrow are fast when planing because they achieve the proper attack or planing angle but require more wind to achieve planing.
Other factors in board design also affect performance in a variety of conditions, particularly in regard to planing. For example, if a board is flat, it will plane in lower wind but tends to ride “hard” under conditions of even a slight chop (e.g. wind driven small waves). If it is large so that it planes in low wind, it is not as fast in higher winds because it will assume too small an angle of attack. If the bottom of the board has a V-shape, it will ride more smoothly but will not plane as fast (e.g. requires more wind to achieve planing). The board will also ride more smoothly if it has more “rocker” (e.g. curvature front-to-rear). It will be faster when not planing and may be faster when planing in high wind due to reduction in wetted area. However, increased “rocker” makes it plane more slowly and requires additional wind for planing due to the decreased angle of attack at the rear which may even cause friction where the bottom surface tries to leave the water. Thus, increased rocker is generally desirable in displacement hulls while decreased, if any, rocker is desirable in planing hulls.
Commercially available boards which are designed primarily to perform in light wind are generally too flat to perform well in higher wind. Such boards are more flat and plane at an angle of attack less than the optimum 4°-7°; thus having increased wetted surface and associated drag.
In this regard, it is known for relatively small motor boats (having a significant degree of rocker) to install trim plates extending behind the transom or stern of the boat which can be deflected slightly downwardly to provide lift at the stern of the boat and thus increase the stern angle of attack when the hull is beginning to plane. The trim plates thus reduce power requirements and smooth the transition between displacement and planing modes of operation. However, it is not practical to use such expedients on a sailboard since control by the operator is impractical.
Further, for both boats and sailboards, such trim plates or hull shaping to the same purpose (which is effectively contrary to the function of rocker), if not properly set for the current speed, can cause an effect known as porpoising. Porpoising is an unstable state in which excess lift at the rear or stern forces the bow lower in the water where rocker causes increased lift at the bow; resulting in an oscillatory pitching action and increased drag. Moreover, with sailboards, some of the deleterious effects of excessive rocker, such as increased angle of attack can be ameliorated by alteration of fore and aft balance at the displacement/planing transition by a suitably skilled operator.
Planing hulls may also be of either the stepped or unstepped types. While the latter has a substantially continuous lower surface, the former, stepped type has an upward step or recess in the bottom surface which is either in front of the center of gravity or very small. This step, under planing conditions at relatively high speed, reduces the wetted surface and associated drag. However, the discontinuity in the shape of the bottom surface also tents to increase drag (for reasons that have not previously been well-understood but intuitively thought to be related to a combination of turbulence and suction behind the step and deeper extension into the water) during displacement mode operation and increase the difficulty of the transition between displacement and planing conditions as well as increasing the power/speed required to reach planing conditions.
Possibly for this reason, stepped bottom surfaces are not generally used for sailboards. Among currently commercially available designs, only the Pro-tech C. A. T., which has an approximately one-half inch step near the rear of the board, provides a stepped bottom surface rather than a single running or planing bottom surface. Further, the step is either completely surrounded by water (during displacement operation) so it only functions as a step in the mainly displacement mode (low speed planing or slower) or completely out of the water (during planing operation).
For a wing having flow across both the top and bottom surfaces, the effect of AR on the lift coefficient, CL, has been determined by Prandtl in 1918 and by experiments to beCL=1.8π(α+β)/(1+2/AR)where α is the angle of attack and β is the wing curvature in the direction of the flow. Thus, it can be seen directly that a reduced aspect ratio reduces lift. Reduced aspect ratio also increases induced drag and reduces the lift to drag ratio. Trailing vortices which cause reduced lift and increased drag for low aspect ratio boards are easily observed and are similar to trailing vortices produced by a wing.
U.S. Pat. No. 5,823,480 to LaRoche and “Wing-grid, A Novel Device for Reduced Induced Drag on Wings” by LaRoche and Palfrey, Fluid Mechanics Laboratory HTL Bruggs-Windisch, CH-5200 Switzerland disclose winglets or a wing-grid (multiple short wings, possibly with free ends much like feathers of a bird, or a grille-work of airfoils much like a multiply slotted aircraft wing) can be used to increase the effective AR and thus reduce the trailing vortices and induced drag. Essentially, a so-called fence at the end of a wing or hydrofoil can increase the lift of the wing/hydrofoil and thus increase the effective aspect ratio of the wing/hydrofoil. However, these reported effects have been confined to environments providing flow on both surfaces and not with planing surfaces.
In summary, while numerous design features of watercraft hull shapes are known for enhancement of efficiency and performance, each such feature and most combinations thereof have tended to narrow the range of conditions under which such enhancement can be realized. These limitations are particularly critical where available power is limited as is the case with sailboards which operate solely under sail power and where the sail area is severely limited by the necessity of being held in place by a human operator, principally by balancing wind force with limited body weight.
Further, good planing performance is of high importance with sailboards since high speed is very desirable in the windsurfing sport and less power is required while planing, as alluded to above. Moreover, the speed increase which occurs when planing is achieved greatly increases apparent wind speed during reaches (sailing generally across or toward the wind), allowing substantial increase in the speed attainable as well as generally increased maneuverability. Nevertheless, known designs of sailboard hulls only support such levels of performance within a limited range of conditions (e.g. wind speed, water surface chop, and the like) while the cost and size of sailboards and other practical considerations effectively prevent alternative use of sailboards of different designs to exploit particular conditions which may prevail at any given time. To date, no single sailboard hull design has been proposed which provides desirable characteristics over a wide range of wind and water conditions, particularly providing stability and ease of control even in heavy winds and chop with the ability to achieve planing in very light wind (e.g. of about six miles per hour or less), to present low drag and high lift over a wide range of speeds and a smooth displacement to planing mode transition and to provide increased efficiency in both the displacement and planing modes of operation to provide higher speed in both modes for given wind speed, particularly by further increasing the effective aspect ratio when planing while maintaining a low physical aspect ratio for increased efficiency in the displacement mode.